1. Field of the Invention
The present invention relates to a process for producing a semiconductor layer on a substrate and an apparatus for producing the semiconductor layer, and particularly to process and apparatus for producing a semiconductor layer of a non-monocrystalline silicon type such as amorphous silicon, amorphous silicon germanium, amorphous silicon carbide or microcrystalline silicon, which is used in solar cells, photosensitive drums for copying machines, image sensors for facsimiles, thin film transistors for liquid crystal display devices, etc. The present invention also relates to a process for fabricating a photovoltaic cell using such a production process.
2. Related Background Art
Amorphous silicon permits the formation of a large-area semiconductor film by a plasma CVD process and so permits the fabrication of a large-area semiconductor device with comparative ease compared with crystalline silicon and polycrystalline silicon.
Therefore, amorphous silicon films are often used in semiconductor devices of which a large area is required, specifically, in solar cells, photosensitive drums for copying machines, image sensors for facsimiles, thin film transistors for liquid crystal display devices, etc.
These devices are larger in the area of a device compared with devices comprising crystalline semiconductors, such as LSI and CCD. In the case of, for example, a solar cell, an area as large as about 30 m.sup.2 per home is required to provide an output of about 3 kW, by which electric power for general home is furnished, when a conversion efficiency thereof is 10%. Therefore, a solar cell module also occupies a considerable large area.
The formation of an amorphous silicon film is generally conducted by a plasma CVD process in which a Si-containing raw gas such as SiH.sub.4 or Si.sub.2 H.sub.6 is decomposed by high-frequency discharge into a plasma state, and a film is formed on a substrate placed in the plasma.
In the case where the amorphous silicon film is formed by the plasma CVD process, high frequency of RF (radio frequency; about 13.56 MHz) has heretofore been generally used.
On the other hand, attention has been paid to plasma CVD using VHF (very high frequency) in recent years.
For example, Amorphous Silicon Technology, pp. 15-26, 1992 (Materials Research Society Symposium Proceedings Volume 258) reports that discharge frequency is changed from RF of 13.56 MHz to VHF, whereby high-density plasma can be obtained to enhance a film-forming rate to a markedly high degree, so that a good deposited film can be formed at a high speed.
U.S. Pat. No. 4,406,765 discloses a high-frequency plasma CVD process in which a direct current (DC) electric field is applied. It is said that a moderate DC electric field is applied together with a high-frequency electric field in the plasma CVD process, whereby a good-quality amorphous semiconductor can be provided.
However, the application of the plasma CVD process using VHF, which is capable of forming a deposited film at a high speed as described above, to deposition of a large-area film has involved the following problems.
Namely, when a large-area flat plate discharge electrode generally used in RF is used to induce uniform discharge over a large area, impedance is hard to be matched in VHF, resulting in difficulty of obtaining uniform plasma on the discharge electrode.
When a rod or radial antenna electrode is used, impedance is matched. However, the balance of an area ratio between the discharge electrode and an opposite electrode, which is substantially 1 in parallel-plate electrodes, is destroyed due to the extremely small area of the discharge electrode. The absolute value of self-bias, which should become smaller in small-area parallel-plate electrodes as frequency increases, becomes greater on the contrary, so that the discharge electrode generates high negative self-bias voltage. In this case, the area of the discharge electrode is small, and so a large-area substrate cannot be place thereon, and high positive voltage against the discharge electrode is applied to the substrate.
As disclosed in U.S. Pat. No. 4,406,765 described above, it is important to apply the moderate DC electric field together with the high-frequency electric field in order to provide a good-quality amorphous semiconductor. It has however been difficult to control the quantity of bias supply power for suitably controlling the DC electric field without causing abnormal discharge such as spark in a discharge chamber or causing dielectric breakdown by charge-up on the surface of the resulting deposited film.
In order to control the DC electric field, it has been known to apply high-frequency bias power in addition to the high-frequency power for decomposing a raw gas by discharge, besides the above-described method by the application of the DC voltage. This method is disclosed in Japanese Patent Application Laid-Open No. 6-232429 and the like. Even in this case, it has been difficult to control the quantity of bias supply power for suitably controlling the DC electric field without causing abnormal discharge such as spark in a discharge chamber or causing dielectric breakdown by charge-up on the surface of the resulting deposited film.
In addition, as a continuous fabrication apparatus of an amorphous silicon type semiconductor device, U.S. Pat. No. 4,400,409 and the like disclose a continuous plasma CVD apparatus using a roll to roll system.
According to this apparatus, plural glow discharge chambers are provided, and a large-area device having semiconductor junctions can be continuously fabricated by arranging the glow discharge chambers along a passageway through which a sufficiently long band-like substrate having a desired width successively passes and feeding the substrate in the longitudinal direction thereof while depositing and forming a semiconductor film of a necessary conductive type in each glow discharge chamber.
As described above, when the continuous plasma CVD apparatus of the roll to roll system is used, the device can be continuously fabricated for a long period of time without stopping the fabrication apparatus, so that high productivity can be achieved.
When DC electric field is applied to plasma in this plasma CVD process of the roll to roll system, however, the same problems as described above have been encountered in VHF in particular.
The roll to roll system has also involved a problem that although there is a plurality of discharge chambers in which a deposited film is formed, DC voltage of different levels cannot be applied to the plural discharge chambers by such a method by applying DC voltage to the substrate side as disclosed in U.S. Pat. No. 4,406,765, since the substrate is continuous and common and is generally conductive, so that a bias voltage level cannot be suitably set according to the kind of a deposited film and discharge conditions in each discharge chamber.